1. Field of the Invention
The present invention relates to a stacked type piezoelectric element and a vibration wave motor.
2. Related Background Art
As an example of electro-mechanical energy conversion elements, there is a stacked type piezoelectric element capable of generating a large deformation distortion and a large force with a low voltage. The stacked type piezoelectric element is disclosed in U.S. Pat. No. 5,770,916, and is used for a vibration wave motor, for example.
The stacked type piezoelectric element of this kind has a laminated structure in which a plurality of piezoelectric layers composed of, for example, piezoelectric ceramics are stacked. On a surface of each piezoelectric layer, inner electrodes composed of an electrode material are formed.
A structure of the stacked type piezoelectric element will be described with reference to FIGS. 7A and 7B. FIG. 7A is a side view of a conventional stacked type piezoelectric element, and FIG. 7B is a diagram of the stacked type piezoelectric element shown in FIG. 7A which is broken down into piezoelectric layers.
As shown in FIG. 7A, a stacked type piezoelectric element 70 has a cylindrical laminated structure in which a plurality of piezoelectric layers are stacked. As shown in FIG. 7B, in a case where the stacked type piezoelectric element 70 has 25 piezoelectric layers, for example, a surface electrode layer 75 is arranged as a first layer thereof, and a piezoelectric layer 71 serving as a sensor layer is arranged as a second layer thereof. From a third layer to a twenty-fifth layer of the stacked type piezoelectric element 70, piezoelectric layers 72 and piezoelectric layers 73 are alternately arranged. A surface of the piezoelectric layer 71 is provided with four-way split inner electrodes S+, B+, A−, and B−. A surface of each of the piezoelectric layers 72 is provided with four-way split inner electrodes AG+, BG+, AG−, and BG−. A surface of each of the piezoelectric layers 73 is provided with four-way split inner electrodes A+, B+, A−, and B−.
The surface electrode layer 75 and the piezoelectric layers 71 to 73 that constitute the stacked type piezoelectric element 70 have an opening at respective center portions thereof, and are formed in a disc shape having the same outer diameter size. The piezoelectric layer 71 is provided with five through-holes 74. The through-holes are holes piercing the piezoelectric layers and filled with a conductive material. Each of the piezoelectric layers 72 and 73 is provided with four through-holes 74. The inner electrodes AG+, BG+, AG−, and BG− of the piezoelectric layers 72 are electrically connected to one another via the through-holes 74 which correspond to an upper piezoelectric layer or a lower piezoelectric layer. However, the piezoelectric layer 72 as the twenty-fifth layer is a bottom layer, so the piezoelectric layer 72 is not provided with any through-holes. The inner electrodes A+, B+, A−, and B− of the piezoelectric layers 71 and 73 are electrically connected to one another via the through-holes 74 which correspond to an upper piezoelectric layer or a lower piezoelectric layer. The surface electrode layer 75 is provided with nine electrodes for allowing the surface electrode layer 75 to be electrically in contact with an outside. Each electrode of the surface electrode layer 75 is electrically connected to the nine inner electrodes AG+, BG+, AG−, BG−, A+, B+, A−, and B− connected via corresponding through-holes 74, and to the inner electrode S+ of a sensor phase.
To the stacked type piezoelectric element 70 having the above-mentioned structure, a polarized process is applied to excite a vibration suitable for a vibration wave motor.
Next, a rod-type vibration wave motor using the stacked type piezoelectric element 70 will be described with reference to FIGS. 8 and 9. FIG. 8 is a longitudinal-sectional view of the rod-type vibration wave motor using the stacked type piezoelectric element 70 shown in FIGS. 7A and 7B. FIG. 9 is a graph showing a relation of a phase difference (θA−S1) between an applied voltage of the inner electrode A+ and an output signal (S1) of the inner electrode S+ to a frequency of the stacked type piezoelectric element, and a relation of an amplitude (VS1) of the output signal (S1) to the frequency of the stacked type piezoelectric element.
As shown in FIG. 8, a rod-type vibration wave motor 80 includes a vibration device 81. The vibration device 81 includes an elastic member 82, an elastic member 83, the stacked type piezoelectric element 70, and a wiring substrate 85, which are all pierced by a shaft 84. The stacked type piezoelectric element 70 is arranged between the elastic member 82 and the elastic member 83. The wiring substrate 85 is arranged between the stacked type piezoelectric element 70 and the elastic member 83. The nine electrodes formed on the surface electrode layer 75 of the stacked type piezoelectric element 70 are electrically connected to corresponding electrode patterns on the wiring substrate 85, respectively. The elastic member 82, the elastic member 83, the stacked type piezoelectric element 70, and the wiring substrate 85 are held and fixed by the shaft 84 and a nut 90 which is screwed into an end portion of the shaft 84.
The elastic member 82 is brought into pressurized contact with a rotor 88 via a spring 86 and a spring support member 87. In this case, one surface of the elastic member 82 is provided with a friction member 82a, and the rotor 88 is provided with a contact member 88a which is brought into pressurized contact with the friction member 82a. The rotor 88 is engaged with a gear 89. The gear 89 is rotatably supported by a fixing member 91 which is fixed to the shaft 84 by a nut 92.
In this case, the inner electrodes A+ and A− of a phase A and the inner electrodes B+ and B− of a phase B, and the inner electrodes AG+ and AG− of a phase AG and the inner electrodes BG+ and BG− of a phase BG, which are opposed to each other, are connected to a ground, respectively. The inner electrodes A+ and A− of the phase A are applied with a high-frequency voltage having a frequency substantially equal to a natural frequency of a vibration device. Further, the inner electrodes B+ and B− of the phase B having a space phase position different from the phase A by π/2(rad) are applied with a high-frequency voltage having the same frequency and a phase electrically different from the phase A by π/2(rad). As a result, the vibration device 81 incorporated with the stacked type piezoelectric element 70 generates two orthogonal bending vibrations. The rotor 88 brought into pressurized contact with the vibration device 81 is frictionally driven to be rotated around the shaft 84. In association with the rotation of the rotor 88, the gear 89 is rotationally driven.
As a frequency (AC frequency) of a high-frequency voltage applied to the inner electrodes, a neighborhood of a resonant frequency of a natural mode of the vibration device 81 is generally selected. However, the resonant frequency of the vibration device 81 changes within a range of several hundreds to several thousands Hz due to a variation in each solid, ambient temperature, loads on the vibration device 81, or the like. In order to drive the vibration device 81 efficiently and stably, the AC frequency of an input voltage needs to be controlled, and a sensing means for monitoring a vibration state of the vibration device 81 should be provided. Thus, the stacked type piezoelectric element 70 is provided with the piezoelectric layer 71 as a sensor layer.
The piezoelectric layer 71 is distorted due to flexion deformity of the vibration device 81, and generates a charge by a piezoelectric effect. The inner electrode S+ provided to the piezoelectric layer 71 is arranged at a position where a phase thereof coincides with that of the inner electrode A+. FIG. 9 shows the relation of a phase difference (hereinafter, referred to as “θA−S1”) between an applied voltage of the inner electrode A+ and an output signal (hereinafter, referred to as “S1”) of the inner electrode S+ to the frequency of this case, and the relation of an amplitude (VS1) of the output signal S1 to the frequency of this case. When the vibration device 81 is driven in a bending vibration mode, the phase difference θA−S1 at a resonant frequency Fr becomes π/2(rad) both in a clockwise direction (CW) and in a counter-clockwise direction (CCW). As the frequency becomes higher than the resonant frequency, the phase difference θA−S1 is gradually deviated from π/2(rad). The amplitude (VS1) of the output signal becomes a maximum value in the vicinity of the resonant frequency, and gradually becomes smaller as the frequency becomes higher than the resonant frequency.
In addition, a vibration wave motor which is more compact than the above-mentioned vibration wave motor is disclosed in Japanese Patent Application No. 2003-134858, for example.
In order to provide a vibration wave motor with a stable performance, Japanese Patent Application No. 2003-164171 discloses a method of deliberately adjusting decay components of the phase A and the phase B as a method of reducing unevenness of a traveling wave, i.e., unevenness of an amplitude of a vibration wave, which is closely related to the performance thereof. In addition to this, proposed is, for example, a method of setting a difference between two resonant frequencies Δf(=fa(resonance frequency of phase A)−fb(resonance frequency of phase B)) to 10 Hz≦fa−fb≦100 Hz.
In recent years, particularly the above-mentioned vibration wave motor is strongly demanded to achieve a smaller size, a higher efficiency, and a higher output in order to be adaptable to a variety of applications.
However, in a case where the stacked type piezoelectric element with the conventional structure is used for a more compact vibration wave motor, larger-than-expected unevenness of traveling wave is caused. To be specific, the vibration device is downsized with respect to the compact vibration wave motor, and the stacked type piezoelectric element is also downsized. As the stacked type piezoelectric element is downsized, the capacitance of the stacked type piezoelectric element becomes smaller, and the electric resonant frequency value (=½/π/≦√LC), which greatly affects the amplitude of each phase, changes to a large extent only by a little difference in capacitance.
For example, in a case where a motor having a mechanical resonant frequency of 60 kHz and a capacitance of 70 nF of each phase is provided with a sensor layer as shown in FIG. 7B, the electric resonant frequency value changes by about 1.2 kHz. On the contrary, when the capacitance of each phase is set to 30 nF, the electric resonant frequency value changes by about 3 kHz, which is twice and a half of the former case. A prerequisite for this case is that a difference between the electric resonant frequency and the mechanical resonant frequency is kept to be substantially equal.
Up to now, it is known that an input power of each phase changes due to the difference between the electric resonant frequency and the mechanical resonant frequency. In other words, an imbalance between the phase A and the phase B is caused due to the capacitance of the sensor phase which is provided in the same phase as the phase A serving as a sensing means for monitoring a vibration state of the vibration device of the stacked type piezoelectric element used for the compact vibration device. Such the imbalance is supposed to be a major cause of the generation of unevenness of the traveling wave.